Liquid crystal displays, and similar displays, have polysilicon thin film transistors (TFTs) for operating each pixel of the display. Each TFT includes a thin layer of polysilicon with a layer of a dielectric material, such as silicon dioxide, over a portion of the polysilicon layer. A gate of a conductive material is over the dielectric layer and across a portion of the polysilicon layer, and metal contacts are provided to the polysilicon layer at each side of the gate to form the source and drain of the TFT. The gate is also generally covered with a layer of a dielectric material, such as silicon dioxide, which is generally doped with either phosphorus and/or boron. Desirably, a layer of an electrical conductor, such as indium-tin oxide (ITO) is connected to the drain of the TFT through suitable metallization. The conductive layer could cover the gate region of the TFT while it serves as the electrode for an individual pixel of the display.
Hydrogenation of the polysilicon TFTs is a known method for improving the current-voltage characteristics of the TFTs. These improvements include reducing leakage current and increasing drive current. This hydrogenation is achieved by introducing hydrogen into the polysilicon layer. Heretofore, this hydrogenation typically has been carried out before the conductive layer is applied over the TFTs. From a manufacturing point of view, it is desirable that the hydrogenation step be performed after the conductive layer is deposited and patterned. If the hydrogenation is performed prior to the deposition of the conductive layer, particularly an ITO conductive layer, the temperature at which the deposition and other subsequent depositions are carried out could cause the hydrogen to be driven out of the polysilicon layer. However, we have found that if the conductive layer is directly exposed to a hydrogenation plasma, the conductive layer may be deleteriously attacked by the hydrogen plasma.